If they upped healing requirements across the board to enforce a greater focus on casting heals and managing MP (largely precluding DPS), we'd probably see even fewer players regularly playing healers, so I doubt SE would take this route.

The good healers are already keeping themselves busy by weaving DPS.

The mediocre healers have their hands full as it is or refuse to step up. They're likely to complain if healing becomes demanding enough to affect the play style of the upper skill tier.

The bad healers struggle to meet minimum requirements and won't be able to hang at all unless they experience some epiphany that helps them realize what they're doing wrong and how to fix it (if they aren't bad because of sheer apathy).